The narrator of "The Murders in the Rue Morgue" is a stranger in a strange land, a man of unknown nationality visiting Paris. While he and Dupin are bonded, he nevertheless often describes Dupin specifically as a Frenchman. The testimony with regard to the murder case arising from the neighbors in the Quartier St. Roch is similarly nationally marked. There's a bunch of French, English, Italian, and Dutch people, all presumably sharing a neighborhood. While Paris is the capital of France, it also comes across, in this story, as the capital of the world, with all kinds of people living side by side. There's no sense of rootedness or unified national culture to be found in the midst of this thriving urban setting.
In such a dense city space, you expect misunderstandings to come up. And they do, literally. None of the witnesses can agree on the language that they hear spoken over their heads as they race into the house in the Rue Morgue, but they all think it's a foreign language. In a neighborhood in which many nationalities of people exist cheek by jowl, there's still one boundary that's maintained absolutely: the animal remains foreign. In other words, whatever his or her nation of origin, everyone in Paris is connected by their common status as reasoning, rational human beings. What needs to be excluded to keep up this status quo is the irrational emotionalism of the Ourang-Outang.
"The Murders in the Rue Morgue" depends on the diversity of modern city life to make its story of a murderous Ourang-Outang plausible.
Dupin's coldness and calculation exclude him from society as much as the Ourang-Outang's extreme emotions do.